1. Field of the Invention
The invention relates to an apparatus for precisely measuring the level of a liquid.
2. Description of the Related Art
Most vehicles, including cars, trucks aircraft and boats include fuel tanks and sensors for detecting the level of fuel in the fuel tank. One common fuel level sensor includes a float that will ride approximately on the surface of the liquid fuel in the fuel tank. Thus, the float will move vertically in the fuel tank as the amount of fuel in the tank varies. The float may be disposed to slide along a detector and the relative position of the float along the detector provides an indication of the level of fuel in the tank. Fuel level sensors that rely upon a float are mechanically complicated, costly and imprecise. Furthermore, a problematic fuel level float is very difficult to repair or replace without removing and disassembling a significant part of the fuel system of the vehicle.
Time domain reflectometers are used to measure the length of a coaxial cable and have been used to measure the level of a liquid. An example of the use of time domain reflectometers to measure a quantity of liquid is shown in U.S. Pat. No. 3,703,829. More particularly, U.S. Pat. No. 3,703,829 relates to a system that mounts a coaxial cable vertically in a tank. The coaxial cable has a center conductor, an outer conductor and an air dielectric between the center conductor and the outer conductor. Electrical signal pulses are generated and applied to the upper end of the center conductor of the coaxial cable. The signals travel down the center conductor and are reflected at the fluid surface. The volume of fluid in the tank is a function of the time delay between the application of the pulse and the receipt of the surface-reflected pulse. The time domain reflectometer technology of U.S. Pat. No. 3,703,829 can work well if the tank and time domain reflectometer can be designed concurrently and if space is not an issue. Most tank level gauges do not rely on time domain reflectometers and cannot be retrofitted easily for the type of time domain reflectometer disclosed in U.S. Pat. No. 3,703,829.
Accordingly, an object of the invention is to provide a fluid level gauge that enables time domain reflectometer technology to be retrofitted into an existing tank.